The present invention generally relates to a method of directly bonding a crystal to a crystal without the interposition of a bonding agent, and to a crystal device which can be used in a mobile communication device such as portable telephone or the like.
The conventional crystal resonator is constructed so that electrodes are oppositely established through crystal blanks worked into a disk shape and a lens shape by mechanical and chemical working methods. Conventionally a crystal resonator to be used in a mobile communication device is called an AT cut crystal resonator whose resonance mode is a thickness shear mode. The resonant frequency of the AT cut crystal resonator is determined by the thickness and elastic constant of the crystal blank. Also, the resonant frequency-temperature characteristics are determined by its cut angle, and shows a tertiary curve. A frequency stability of .+-.1.0 ppm is obtained in the temperature range of 0.degree. through 50.degree. C. (see, for example, `Piezo-electricity`; edited by Walter Guyton CADY, Dover Publ., Inc.) by the control of the cut angle. A method (see Japanese Laid-Open Patent Publication No. 55-138914) of causing a material of a different line expansion coefficient on a crystal basic plate or engaging it (see Japanese Laid-Open Patent Publication Nos. 55-158718 and 56-61899) is proposed as a method of increasing the frequency stability.
A crystal piece is shaped (convex) so that its central portion may become thicker than the environment portion like an optical lens as shown. A Q value is increased with the elastic vibrations being contained within the central portion when the central portion of the electrode is made thicker (see, for example, Japanese Laid-Open Patent Publication No. 2-260910).
A method of partially thinning the crystal blank by a chemical etching method, a method of using a fundamental mode and a overtone mode at the same time, or other methods have been considered as methods of outputting a plurality of vibrations from one crystal resonator.
The crystal oscillator is an oscillator designed to stabilize frequencies using crystal oscillator and having a Q of approximately 100,000. Therefore, the output frequency-temperature characteristics of the crystal oscillator depend upon the resonant frequency-temperature characteristics of the crystal oscillator. When the conventional AT cut crystal resonator is used, the frequency stability of .+-.1.0 ppm is obtained in the temperature range of 0.degree. through 50.degree. C. A frequency stability of .+-.1.0 ppm at -30.degree. C. through 80.degree. C. is demanded of a crystal oscillator to be used in a portable telephone. A temperature compensating circuit is required so as to satisfy this demand. A crystal oscillator with a temperature compensating circuit being provided in it is called a temperature compensated crystal oscillator whose type can be chiefly divided into analog systems and digital systems.
The conventional analog temperature compensated crystal oscillator is composed of temperature sensing elements such as a thermistor, capacitor elements such as capacitors, varactor diodes, and resistors so as to compensate for changes due to temperature of the reactance (resonant frequency) of the crystal resonator with the reactance of the temperature compensating circuit being changed for stabilizing the output frequency.
The conventional digital temperature compensated crystal oscillator is stabilized by data stored in advance by the corresponding operation of the output frequency of the digital control crystal oscillator composed of a variable reactance circuit, an oscillating circuit and a crystal oscillator. A method of outputting fundamental frequencies and higher order overtones or spurious overtones with one crystal blank sheet so as to suppress the temperature difference caused by the difference in location between the temperature sensor and the crystal oscillator, and the difference in the thermal time constant (see, for example, Japanese Laid-Open Patent Publication No. 2-170607 and Japanese Laid-Open Patent Publication No. 2-174407).
The conventional crystal filter has two sets of opposite electrodes adjacently formed onto both faces of the crystal blank. One set is an input electrode and the other set is an output electrode. When an electrical signal is inputted into an input electrode, the elastic vibration of the resonant frequency is excited into a crystal blank in a region where the input electrode is formed. The elastic vibrations are propagated into the crystal blank and reach a region where the output electrode is formed. As a result, an electric field is caused in a region where the output electrode is formed, and the electrical signals are output from the output electrode. Namely, the filter becomes a band-pass filter of a resonant frequency to be determined by the thickness of the crystal blank and the elastic constant thereof (see, for example, Proc. 39th Ann. Frequency Control symposium, pp. 481 -485 `VHF MONOLITHIC CRYSTAL FILTERS FABRICATED BY CHEMICAL MILLING`).
A three-dimensional processing is hard to effect in only the processing of the crystal blank simply by such mechanical and chemical polishing operations as described hereinabove, with a problem that the processing time is long.
There is a restriction in improvements of stability of the crystal resonator only by adjusting a cut angle. These materials become resistors which interfere with the elastic vibrations of the crystal in a method of using a different material on the crystal basic plate or engaging it, with a problem that the Q of the crystal resonator is deteriorated.
A method of convexly processing the crystal blank as a method of increasing the Q has problems in that the accuracy of the resonant frequency is lowered. A method of evaporating electrode films many times such that each film has a different size has problems in that more time is required.
A method of outputting two vibrations using a fundamental mode and a overtone mode of one crystal blank sheet cannot separately set the respective resonant frequencies and the temperature characteristics.
Separate compensating circuits are required for high temperature use and a low temperature use in an analog temperature compensated crystal oscillator using a thermistor or the like, with a problem in that the number of circuit parts is increased, and an adjusting operation becomes complicated, thus resulting in a higher cost.
The digital temperature compensated crystal oscillator is provided with separate crystal resonators for temperature sensor and oscillation use. Proper temperature compensation cannot be effected if the temperature difference caused by the difference in the thermal time constant between them, the difference in their location and so on are not suppressed, and large capacity memories are required, and adjusting costs become higher. A method of outputting the fundamental modes, higher order overtones or spurious modes with one crystal resonator sheet becomes respectively essential to excite, across the wide temperature ranges, independently, selectively and stably so as to suppress the temperature difference between the temperature sensor and the crystal resonator. Therefore, the designing of the crystal vibrator, the bias of the oscillating circuit and the feedback capacitor must be carefully determined. The bias and the feedback capacitor of the oscillation circuit must be temperature compensated as the case may be, with problems in that the circuit scale is too large and disordered to be fabricated.
The crystal filter has problems in that the stability of the frequency-temperature characteristics are lower, and it is difficult for the size to be made smaller because of its plane construction.